1. Field of the Invention
The invention relates to geared, multiple-ratio power transmission mechanisms having dual clutches to permit selective engagement and disengagement of ratio change clutches for operation in a given ratio while preselecting ratio change clutches for an adjacent ratio during a shift sequence in separate speed ratio ranges.
2. Background Art
Conventional automotive vehicle powertrains typically have multiple-ratio transmission mechanisms that establish power delivery paths from an engine to vehicle traction wheels. Adequate powertrain performance for heavy-duty or medium-duty powertrains requires more than one range of gear ratios so that the speed and torque characteristic of the engine will provide optimum traction wheel power throughout the operating speed range for the vehicle. Ratio range gearing, therefore, typically is used with a transmission mechanism in such high torque capacity powertrains so that sufficient overall gear ratio coverage is provided. The overall gear ratio coverage will be equal to the product of the number of gear ratios for the transmission and the number of gear ratios in the ratio range gearing.
One class of transmission mechanisms in automotive vehicle powertrains includes countershaft gearing having power delivery gears journaled on a transmission mainshaft and countershaft gear elements journaled on a countershaft arranged in spaced, parallel disposition with respect to the mainshaft. The countershaft gear elements typically mesh with the gears mounted on a mainshaft axis. Operator-controlled clutches, which may be either positive drive dog clutches or synchronizer clutches, selectively connect torque transmitting gears on the mainshaft axis to the mainshaft, thereby establishing a power delivery path from the engine to the traction wheels. In other countershaft transmission mechanisms, the dog clutches or the synchronizer clutches may be mounted on the countershaft axis to selectively engage countershaft gear elements with torque transmitting gears on the mainshaft.
It is known design practice to use a dual clutch arrangement for selectively connecting the engine to first and second torque input gears, sometimes called headset gears, of a countershaft transmission mechanism. An example of a dual clutch countershaft transmission mechanism of this type may be seen by referring to U.S. patent application Ser. No. 10/983,531, filed Nov. 8, 2004, entitled “Dual Clutch Assembly For A Heavy-Duty Automotive Powertrain.” That application is assigned to the assignee of the present invention.
A dual clutch arrangement makes it possible for a countershaft transmission mechanism to be power-shifted from one ratio to another. As one of the dual clutches is engaged, the other is disengaged. The engaged clutch will establish a power delivery path through the countershaft gear elements and through the main transmission gears as one or more of the ratio change clutches are engaged. A ratio change clutch for a main transmission gear, or a countershaft gear element that is not involved in a given selected power flow path, can be pre-engaged in preparation for a ratio change to an adjacent ratio. When a ratio change is initiated, the ratio change clutch for one main gear or for one countershaft gear element is disengaged, and a power flow path is established by the pre-engaged ratio change clutch for a second main gear or a second countershaft gear element. The dual clutches of the dual clutch arrangement are alternately engaged and disengaged (“swapped” or “traded”) thereby providing a smooth transition from one ratio to an adjacent ratio in a seamless fashion.
If a multiple gear ratio power transmission mechanism of the type previously described is intended for use in a powertrain for a medium-duty or heavy-duty vehicle or truck, an increased number of gear ratios is required throughout the engine speed range. Typically, an increased number of gear ratios is achieved by combining a two-speed range gearing arrangement at a torque output portion of the multiple gear ratio transmission. Although a two-speed range gearing arrangement is typical, a range gearing arrangement adapted for an increased number of ratios can be used if a particular application for the vehicle requires broader ratio coverage. A two-speed range gearing arrangement will double the number of gear ratios available in the powertrain. If a multiple ratio transmission mechanism has dual clutches, it can be power-shifted between ratios without torque interruption between the engine and the vehicle traction wheels.
If the powertrain includes multiple speed range gearing in a torque flow path from the multiple ratio transmission mechanism to the vehicle traction wheels, it is necessary with known heavy-duty or medium-duty powertrains for the torque flow path to be interrupted during a transition from one range to the other. That torque interruption may deteriorate the shift quality because of inertia forces that are created. Further, the time needed to execute a ratio change in the range gearing increases the time needed to execute a shift between the highest overall ratio for one range to the adjacent lowest overall ratio for another range.
An example of a multiple range gearing arrangement with an eight-speed transmission is described in a publication entitled “Automotive Handbook,” Third Edition, published by Bosch in 1993, page 544. The overall number of gear ratios in the transmission described in the Handbook is sixteen, which is twice the number of ratios available in the transmission gearing.